An organic compound is brought into an excited state by absorbing light. Through this excited state, various reactions (such as photochemical reactions) are caused in some cases, or luminescence is generated in some cases. Therefore, various applications of an organic compound have been made.
As one example of the photochemical reactions, a reaction of singlet oxygen with an unsaturated organic molecule (oxygen addition) is known (see Non-Patent Document 1: Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics vol. 38, L1502 to L1504, 1999, for example). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen is generated, which can lead to an oxygen addition reaction. In this case, a compound capable of forming the triplet excited molecule is referred to as a photosensitizer.
As described above, in order to generate singlet oxygen, a photosensitizer capable of forming a triplet excited state by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition, and a triplet excited molecule is not easily generated. Therefore, as such a photosensitizer, a compound in which intersystem crossing from the singlet excited state to the triplet excited state easily occurs (or a compound which allows the forbidden transition of photoexcitation directly to the triplet excited state) is required. In other words, such a compound can be used as a photosensitizer and is useful.
Also, such a compound often emits phosphorescence. The phosphorescence is luminescence generated by a transition between energies of different multiplicity and, in an ordinary organic compound, the phosphorescence indicates luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, luminescence generated in returning from a singlet excited state to the singlet ground state is referred to as fluorescence). Application fields of a compound capable of emitting phosphorescence, that is, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element using an organic compound as a light-emitting substance.
This light-emitting element has a simple structure in which a light-emitting layer containing an organic compound that is a light-emitting substance is provided between electrodes. This light-emitting element has been attracting attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. In addition, a display device using this light-emitting element is superior in contrast and image quality, and has wide viewing angle.
The light emission mechanism of a light-emitting element in which an organic compound is used as a light-emitting substance is a carrier injection type. That is, by applying voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make the light-emitting substance excited, and light is emitted when the excited state returns to the ground state. As the type of the excited state, as in the case of photoexcitation described above, a singlet excited state (S*) and a triplet excited state (T*) are possible. Further, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
As for a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound), luminescence from a triplet excited state (phosphorescence) is not observed but only luminescence from a singlet excited state (fluorescence) is observed at a room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when the phosphorescent compound described above is used, the internal quantum efficiency can be improved to 75 to 100% in theory. That is, a light emission efficiency that is 3 to 4 times as high as that of the fluorescence compound can be achieved. For these reasons, in order to achieve a highly-efficient light-emitting element, a light-emitting element using a phosphorescent compound has been developed actively recently (for example, see Non-Patent Document 1: Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics vol. 38, L1502 to L1504, 1999; and Non-Patent Document 2: Chihaya Adachi, and five others, Applied physics Letters vol. 78, No. 11, 1622 to 1624, 2001.).